Implantable hearing aids are used in many applications to rehabilitate hearing impairments of different kinds where conventional air conduction hearing aids will give a less successful treatment result, or are not feasible. One type of implantable hearing aid is the bone conduction hearing aid where a vibrator that directly stimulates the skull bone is fully implanted beneath the skin. This kind of hearing aid is prescribed to patients who are suffering from one or two sided chronic ear infections or a congenital/acquired malformation of the outer and/or middle ear or single-sided deafness. This type of implantable bone conduction hearing aid is often called Bone Conduction Implant (BCI) see Håkansson et al. 2010 and FIG. 1.
Another type of implantable hearing aid which is also illustrated in FIG. 1 is a Cochlear Implant (CI). Cochlear Implants are used for patients suffering from considerably reduced neurogenic function with a substantial reduction of hair cells in the inner ear. The Cochlear implant stimulates the basal cells of the basilar membrane (BM) electrically inside the inner ear by means of an array of electrodes which are introduced inside the cavity of the inner ear along the basilar membrane where these cells are located (see e.g. U.S. Pat. No. 4,063,048). A similar kind of implantable hearing aid is a so called Auditory Brainstem Implant (ABI) which is also illustrated in FIG. 1. Auditory Brainstem Implants are used when the auditory nerve (HN) which conducts sound information from the inner ear to the brain is damaged and then the electrodes are applied to the brainstem (see e.g. U.S. Pat. No. 7,797,029).
Finally, there is a type of implantable hearing aid which has a mechanical loudspeaker (T) connected to one of the ossicles of the middle ear or to the round window (RW) membrane which is also illustrated in FIG. 1. This kind of hearing aid is often called Middle Ear Implant (MEI) and is prescribed to patients for whom the ordinary air conduction hearing aid is insufficient (see e.g. U.S. Pat. No. 5,624,376).
Also belonging to the group of MEI are other implants which directly or indirectly stimulate the fluids of the inner ear mechanically e.g. DACS (Direct Acoustical Cochlear Stimulator from Cochlear Corp. Australia) and MET (Middle Ear Transducer from former Otologics, USA) provided they are operated via an inductive link through intact skin (DACS and MET are not shown In FIG. 1).
These types of implantable hearing aids all have in common that they have an external unit (EU) which is sometimes denoted “sound processor” or “audio processor” as well as an implanted unit (IU), and the sound energy is transmitted from the external unit to the implanted unit via an inductive link which allows the skin and soft tissues covering over the implant to remain intact. The inductive link consists of a circular transmitter coil (TC) located in the external unit and a similar circular receiver coil (RC) which is located in the implanted unit. In order for the external unit in implantable hearing aids to remain firmly fixed in the correct location when they are used by the patient two retention magnets are typically used. Typically one external retention magnet (EM) is seated in the centre of the transmitting coil (TC) in the external unit and one implanted retention magnet (IM) is seated in the centre of the receiving coil (RC) in the implanted unit such that the coils are thereby axially centred with respect to each other. In some cases several magnets may be used on each side of the skin respectively, and in some topologies they may have different axes of magnetic polarity, but the principle function is to provide retention of the external unit to the implanted unit and to centre the coils above each other to provide the best possible inductive transmission of sound energy. FIG. 1 also illustrates that the signal, which is received by the receiving coil, is processed in an electronic unit (E) before the signal is finally driving the actuating units, respectively. In the BCI, the electronic unit (E) may advantageously be integrated in the actuating unit, together with the vibration transducer unit, encapsulated in a titanium casing.
The external unit contains, besides the transmitter coil and retention magnet, also active components such as microphone (M), signal processor (SP), driving circuits for the inductive link (D) as well as a battery (B) for the power supply. As shown in FIG. 1, the battery may easily be dismantled/replaced (see double headed arrow) and when the battery is removed the power supply is also interrupted, which otherwise directs the power to the active electronic circuits via a bipolar battery contact (BC). In order not to consume power when the hearing aid is not in use, there is also a mechanical switch (S) by means of which the patient may turn off the power, or the patient will have to remove the battery from the battery contacts.
One drawback with implantable hearing aids is that they have high electric power consumption as there are inevitable losses related to the inductive link during transmission of energy through the intact skin. As the carrier wave of the inductive link must remain full-on the whole time disregarding sound levels the useful lifetime of the battery may to appear short as compared to battery life time in a conventional hearing aid. The battery present in the external unit lasts 5-7 days for BCI and MEI (depending on how many hours a day the device is used) while for the CI and ABI the useful lifetime of the battery is even shorter as these also have implanted electrical circuits which have a substantial power consumption. It is therefore of utmost importance that the devices are turned off when they are not in use, e.g. at night, or temporarily when it is not necessary to listen via the implant not to consume battery capacity unnecessarily.
It is also important to stress that implantable hearing aids, which make use of a tuned inductive link for transmitting the signal and energy, consume considerably more power if they are not positioned above the implanted unit. This occurs as the tuned circuit for the carrier wave generated in the external unit is no longer tuned to the actual load impedance when it is removed from the head of the patient. Due to changes in the impedance conditions of the induction link the output stage will therefore require even more power than if it is attached to the head of the patient. It can be mentioned that the external unit may be split into two parts where only the transmitting coil (TC) and the external retention magnet (EM) are located in situ on top of the implanted unit (see the parallel dotted line which divides the external unit in two parts in FIG. 1). The remaining part of the external unit is advantageously located in a different housing worn on the pinnae of the outer ear (typically like a behind-the-ear device). This solution is typically used in CI and ABI as they have more than one battery and are therefore heavier than would be suitable to be carried by the retention magnets (would require stronger magnets which may give rise to circulation problems in the soft tissue which will be exposed to a higher pressure).
Implanted hearing aids often use a manual mechanical switch (S) to turn on/turn off the power consumption as shown in FIG. 1 (prior art). As manually activated switches require a relatively large amount of space and may be difficult to handle, some implantable hearing aids have instead made use of the fact that the battery can be removed or moved enough to cut off the power supply through the battery contacts (BC), see double headed arrow in FIG. 1. Both methods require an effort from the patient to turn on/off the power consumption. Compared to conventional air conduction hearing aids and bone anchored hearing aids the patient will not get an indication that the external device is turned off (conventional hearing aids will emit a squeaking sound if they are not turned off and not fitted to the patient). Many elderly have impaired dexterity and may have motoric difficulties to use a small manually activated power switch or to visually inspect whether a mechanical switch is turned “on” or “off”.
One way to accomplish an automatic turning on/off of the power supply is to make use of the changes in the static magnetic flux density which occurs when the external unit is attached. In such a solution a magnetically actuated “reed relay” can be placed at a location so that the relay will be activated when the external unit is fixed in its position and is deactivated when the external unit is removed, see for example U.S. 2005/0033383. With “reed relay” is meant a relay that is activated by a static magnetic field that will exceed/fall below a certain level. When the external unit is fixed in its location the magnetic flux density will increase locally in certain areas and the “reed relay” is therefore advantageously placed in such an area so that the relay will detect this change in magnetic flux density and thereby turn on the power consumption. As an alternative to a reed relay the changes in the static magnetic flux density can be detected by using a Hall sensor. This sensor is thus sensing the static flux and produces a proportional electric voltage. A control circuit can then amplify this quite small electric voltage and at an appropriate selected voltage level from the Hall sensor, the control circuit can activate a relay that switches the power on and off, see for example U.S. 2011/0112607 A1 or U.S. 2005/0105752 A1. All these methods of controlling the power consumption has been considered to be relatively sensitive, especially as the external unit has a permanent magnet in place although the unit has been removed from the patient. This means that the relay only can make use of a change in the static magnetic flow at engagement of the external unit to the implanted unit in order to activate the power supply. Also, a solution using a Hall sensor requires additional electronic components to activate a relay that switches on/off the power from the battery. Such solutions may therefore be both bulky and costly.
Therefore, there is a need for a more inexpensive and reliable alternative device that automatically, directly or indirectly, activates the power supply in the outer unit when it is fitted to the patient and which inactivates or turns off/reduces the power consumption when the external unit is removed from the patient.